Strong opaque materials have recently become desirable for use in electronic casings and other packaging applications, such as cosmetic packaging. For instance, opaque white and black glass-ceramics, such as white beta-spodumene glass-ceramics, are currently being used in smartphone housings. While white and black casings have been popular, there is growing consumer demand for a full palette of opaque, colored glass-ceramics with improved strength for electronic casings and other applications.
Opal glasses with a wide spectrum of colors are commercially available. However, opal glasses tend to suffer from a trade-off between opacity and durability. The precipitation of opalizing phase(s) in the glasses can produce microstructures with an inhomogeneous distribution of large precipitates, which is not conducive to toughness or smooth surfaces. Opaque glass-ceramics, such as lithium disilicate (Fotoceram®), cordierite (radomes), nepheline (Centura®), beta-spodumene (Corning Ware®), and beta-quartz (Visions®), are also known in the art. However, these glass-ceramics are currently limited in terms of their color palette, particularly in terms of obtaining bright colors.
Color is traditionally imparted to glass-ceramics through glazes, which are, in general, fritted materials designed to sinter, flow, and crystallize in one step at low temperatures to form a uniform coating on the glass-ceramic. Color in glasses and glass-ceramics, as opposed to color obtained by applying a glaze, is generally obtained through the addition of a transition metal or rare earth oxide to the batch composition. The color obtained using such oxides depends on various factors, such as the oxidation state of the ion, the coordination of the ion, and/or the nature of the surrounding ions. In glass-ceramics, the transition metal or rare earth ions can partition into the main crystalline phase, the residual glass phase (different from the starting glass), or a secondary (accessory) amorphous or crystalline phase. Thus, the color of the cerammed material (glass-ceramic) can be different from that of the uncerammed material (glass) and may change depending on the ceramming schedule. Colorants have been incorporated into beta-spodumene (opaque) glass-ceramics to produce earthtone products (e.g., beige Corning Ware®), but it is difficult to obtain opaque glass-ceramics in bright colors.
Accordingly, it would be advantageous to provide opaque glass-ceramics which can be doped with colorants to produce bright colors and which can also be chemically strengthened by ion exchange. Additionally, it may also be advantageous to provide a doped spinel-type crystal phase in a nepheline glass-ceramic to produce a strong, opaque, colored product which can be used in various packaging applications, such as electronic casings and cosmetic packaging.